Antibody therapy with rituximab represents a major therapeutic advance in Non-Hodgkin's lymphoma (NHL). However, its benefit to other diseases such as chronic lymphocytic leukemia (CLL) is modest. Several studies have established the importance of the innate immune system to rituximab's efficacy in NHL, whereas its contribution in CLL is unclear. Project 1 focuses on translational development of two immune-based therapies for CLL to enhance the contribution of innate immune system to tumor cell killing. The first therapy is a small modular immune pharmaceutical (SMIP), an engineered peptide with variable regions directed toward the B-cell antigen CD37. This CD37 SMIP effectively recruits natural killer cells to promote tumor cytotoxicity, and also generates an apoptotic signal that is superior to monoclonal antibodies such as rituximab and alemtuzumab currently utilized in the clinic. The second therapy is recombinant IL-21, the first clinically available member of the y-receptor cytokine family. IL-21 potently activates the innate immune system while generating a direct apoptotic signal toward CLL and other NHL cells. Of great interest is that IL-21 activates STAT1 in both NK cells (shown by projects 3 and 4) and CLL cells (shown by projects 1 and 4), but generates activation signals in NK cells and apoptotic signals in CLL cells. Additionally, our data shows that IL-21is different from other y-receptor family members in that it does not expand T-regulatory cells that dampen innate immune cell activation. With significant dependence upon projects 2, 3, and 4 of this proposal, our specific aims include: 1) To study the mechanisms by which CD37 SMIP mediated cytotoxicity occurs and how to optimize its use for clinical trials; 2) To pursue studies of IL-21in CLL to determine both mechanism(s) of apoptosis in CLL and also how this cytokine enhances killing of lymphoid tumor cells; and 3) To pursue clinical development of IL-21 in patients with CLL and related diseases. This will be accomplished by performance of phase l/ll clinical trials of IL-21with fludarabine and rituximab in CLL and related diseases that include detailed pharmacodynamic studies to asses the affect of IL-21on innate immune effector cells and primary tumor cells. Through this work, we hope to identify the relevant mechanisms by which CD37-SMIP and IL-21mediate cytotoxicity to promote effective clinical translation of each agent for lymphoid malignancies